The present invention refers to a method for synchronizing the counter of an electric control device with the zero position, defined by a mechanical stop, of a pointer of an indicating instrument which is driven via a stepdown gearing by a stepping motor having two windings perpendicular to each other and a two-pole rotor. The two windings are connected alternately to a source of voltage and to a measuring mechanism for determining the voltage induced in the winding not traversed by a current, the pointer being moved step-wise up to the mechanical stop and the counter being set to zero in the absence of an induction.
Such a method is described in German OS 39 21 462. As a result of the interpositioning of the gearing, no dependable conclusions can be derived from the position of the rotor in the stepping motor as to the position of the pointer in the indicating instrument. That position is represented only by the counter of the electronic control device, the reading of which, however, may be disturbed for various reasons. In order that no erroneous indications can result therefrom the counter must be constantly resynchronized with the position of the pointer, for which purpose, in the known method the zero position of the pointer defined by a mechanical stop is used in combination with a test program in which the stepping motor is controlled for movement of the pointer in the direction of the zero position until the end of movement of the pointer is detected by the absence of an induced voltage in the winding of the stepping motor which is not traversed at the time by current for the driving of the pointer. The counter can then be set to zero or to the value corresponding to the zero position of the pointer.
In accordance with this prior method, the windings of the stepping motor are alternately acted on by the full drive voltage and connected to a measuring mechanism. With two windings arranged at right angles to each other, there are obtained hereby 90.degree. steps of the two-pole rotor. Since the test method is based on the evaluation of the induced voltage, the individual rotor steps must have a given minimum size in order for the induced voltage to be sufficiently large to be able to distinguish reliably from an absence of the induction.
The known method operates satisfactorily, but it is still considered capable of improvement. For each revolution of the rotor, four switching and four test routines are present in the case of 90.degree. steps, which is too complicated from a circuit standpoint.